Belgrade — As the U.S. administration considers going to war with Iraq, concerns are emerging that Baghdad has been studying the low-tech countermeasures that Yugoslavia used to foil U.S. airstrikes against its military in 1999.

"That's a matter of serious and legitimate concern," said retired General Wesley Clark, who, as NATO commander, led the 78-day bombing campaign aimed at expelling Yugoslav forces from the mainly ethnic Albanian region of Kosovo, where they were engaged in a campaign of ethnic cleansing.

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Yugoslav tactics that worked against NATO

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NATO prevailed by destroying infrastructure and government buildings in Yugoslavia — but it did little real damage to the Yugoslav military in Kosovo.

Before he was ousted in October, 2000, president Slobodan Milosevic co-operated closely with Iraqi President Saddam Hussein's regime. Yugoslav advisers helped revamp Baghdad's air-defence system, and officers of Iraq's Air Defence Command toured Yugoslav bases to study the Kosovo war.

"The war (in Kosovo) proved that a competent opponent can improvise ways to overcome superior weaponry, because every technology has weaknesses that can be identified and exploited," said Cedomir Janjic, an air force historian.

On Tuesday, the U.S. Embassy in Belgrade confirmed that a group of U.S. military experts was in Yugoslavia to determine what benefits Mr. Hussein's military had derived from its co-operation with the Milosevic regime.

Gen. Clark identified several ways in which Yugoslav experience could prove valuable to the Iraqis.

The most significant, he said, was the ability of Yugoslavia's air defences to foil NATO electronics by using different radar frequencies and profiles, and by using "passive tracking" systems that do not give off radiation.

Despite NATO's air supremacy, it never succeeded in knocking out the air defences. They remained a potent threat throughout the conflict, forcing attacking warplanes to altitudes above 15,000 feet, where they were safe from surface-to-air missiles but far less effective in a ground-attack role.

"We were always aware we were being tracked and monitored by them," Gen. Clark said.

NATO won the war in June, 1999, following Mr. Milosevic's decision to withdraw his largely intact army from Kosovo, and after the extensive destruction of bridges, government buildings and other infrastructure targets throughout Yugoslavia.

In contrast, the effects of heavy bombing on the Yugoslav forces in Kosovo were minimal. British ordnance experts who inspected the battlefields after the war determined that only 14 tanks and a handful of armoured vehicles were destroyed in nearly three months of bombing.

The Yugoslavs had dispersed their heavily camouflaged units, thus conserving their assets for the expected alliance ground assault, and used decoys and other mock targets to deceive the attackers.

Iraq was quick to pursue insight from that conflict.

Teams of Iraqi intelligence officers rushed to Yugoslavia in the aftermath of the war to visit command centres and air-defence sites. Many toured Belgrade's Aviation Museum, inspecting destroyed drones, cruise missiles and the remnants of U.S. F-16 Falcon and F-117 Nighthawk stealth fighters.

"Although they wore civilian suits, it was obvious they were Iraqi military," curator Drasko Kostic said.

Meanwhile, Yugoslav technicians were reportedly upgrading Iraq's fibre-optics communications network, allowing commanders real-time control of all units. They modified launchers of SA-6 surface-to-air missiles with optical tracking equipment to allow them to hit targets without using ground guidance radars, and added fuel cells to SA-3 missiles to extend their range to reach high-flying U-2 spy planes.

Over Iraq, U.S. and British pilots enforcing no-fly zones soon noticed a new aggressiveness in the air defences, which began challenging them on a daily basis. Although numerous command bunkers and missile batteries were hit in retaliatory strikes, the Iraqis also managed some successes by downing reconnaissance drones and damaging a U-2.

Gen. Clark said that Yugoslav advisers had enabled the Iraqis to reduce the "effects of weaponry" and passed on "what works and what doesn't in the art of camouflage."

He noted that the Yugoslavs had demonstrated great skill at hiding their armour, guns and infantry in towns and villages.

"That will certainly be of great interest to the Iraqis," he said. "We shouldn't be surprised to find Iraqi forces in mosques, schools and homes."

The White House is said to have settled on a war plan calling for massive air strikes on air defences and key military facilities. But this could quickly unravel if Mr. Hussein's commanders — like Mr. Milosevic's — shield their forces from the strikes and engage the invaders in a long and bloody ground war in cities.

Analysts say the parallels with Kosovo are far more relevant to a possible conflict than the much-touted victory against the Taliban, arguably the most primitive army in the world.

"We realize that a conflict with Iraq will not be like ... Afghanistan," said retired Rear Admiral Stephen Baker of the Center for Defense Information in Washington. "Our tactics should be driven by what we learned in Kosovo."

Yugoslav tactics that worked

An overview of tactics employed by the Yugoslav army to limit the effectiveness of the NATO air strikes:

• Yugoslav air defences tracked U.S. stealth aircraft by using old Russian radars operating on long wavelengths. This, combined with the loss of stealth characteristics when the jets got wet or opened their bomb bays, made them shine on radar screens.
• Radars confused precision-guided HARM and ALARM missiles by reflecting their electromagnetic beams off heavy farm machinery, such as plows or old tractors placed around the sites. This cluttered the U.S. missiles' guidance systems, which were unable to pinpoint the emitters.
• Scout helicopters would land on flatbed trucks and rev their engines before being towed to camouflaged sites several hundred metres away. Heat-seeking missiles from NATO jets would then locate and go after the residual heat on the trucks.
• Yugoslav troops used cheap heat-emitting decoys such as small gas furnaces to simulate nonexistent positions on Kosovo mountainsides. B-52 bombers, employing advanced infrared sensors, repeatedly blasted the empty hills.
• The army drew up plans for covert placement of heat and microwave emitters on territory that NATO troops were expected to occupy in a ground war. This was intended to trick the B-52s into carpet-bombing their own forces.
• Dozens of dummy objectives, including fake bridges and airfields were constructed. Many of the decoy planes were so good that NATO claimed that the Yugoslav air force had been decimated. After the war, it turned out most of its planes had survived unscathed.
• Fake tanks were built using plastic sheeting, old tires, and logs. To mimic heat emissions, cans were filled with sand and fuel and set alight. Hundreds of these makeshift decoys were bombed, leading to wildly inflated destruction claims.
• Bridges and other strategic targets were defended from missiles with laser-guidance systems by bonfires made of old tires and wet hay, which emit dense smoke filled with laser-reflecting particles.
• U.S. bombs equipped with GPS guidance proved vulnerable to old electronic jammers that blocked their links with satellites.
• Despite NATO's total air supremacy, Yugoslav jets flew combat missions over Kosovo at extremely low altitudes, using terrain to remain undetected by AWACS flying radars.
• Weapons that performed well in Afghanistan — Predator drones, Apache attack choppers and C-130 Hercules gunships — proved ineffective in Kosovo. Drones were easy targets for 1940s-era Hispano-Suisa anti-aircraft cannons, and C-130s and Apaches were considered too vulnerable to be deployed.